The islands will have been decorated accordingly to the holiday since late in the evening! Till the following morning, the lights will be off for the most part in all of the buildings, leaving just candles to lit the place.
And that's not all. During that time, odd things will happen all over the place. They might be a bit more startling/scary - maybe one of the decorations will let out a shrilly scream when your character passes by, maybe a tree root will grab onto your character's ankle, maybe the shadows projected by the candles will resemble ghosts or other creatures, etc. Luckily none of these things will last for too long. The events happening could also be plain silly, and more in the "trick" aspect of trick-or-treating. Things like getting eggs thrown at from seemingly nowhere, or finding their rooms full of toilet paper, etc. These will tend to affect the grouchier characters more than the others.
You guys can decide on what will happen, so be creative o/ And, while the decorations are set for everyone, the smaller happenings aren't mandatory. As always, feel free to ask any questions.